The goal of this proposal is to test the hypothesis that integrins are involved in the initiation of osteoblast cell differentiation. Our first goal is to determine which alpha-subunit is responsible for initiating the differentiation of osteoblasts. We will continue on-going studies aimed at determining the integrin species expressed during the development of hard tissues in vivo. Once the species of integrin alpha- subunits that are expressed in hard tissues in vivo are known, we will use a series of molecular approaches to determine which integrin alpha- subunit is involved in developmental processes. We will up-regulate the expression of integrin alpha-subunits (a) by the use of integrin expression vectors, and (b) by the isolation of mutant cell lines which are altered in the expression of selected integrins. Second, we will employ agents that selectively inhibit integrin function (synthetic peptides and neutralizing antibodies). Emphasis will be placed on expression vector studies. This series of experiments should lead to the identification of the integrin alpha-subunit that is responsible for the initiation of osteoblast cell differentiation. The studies proposed above are modelled after the myoblast system in which (a) competence for differentiation is determined by the expression of the MyoD gene and (b) initiation of cell differentiation takes place following transmission of an integrin transduced signal. Our second goal is to examine the integrin signal transduction pathway in osteoblasts. We propose to study the effects of ligand-receptor binding upon signal transduction. Second, we will examine a recently described "adhesion plaque" tyrosine protein kinase system that conveys signals from integrins. third, we will determine the transcription factors that respond to integrin transduced signals. The studies proposed should define the role of integrin signal transduction mechanisms in osteoblast cell differentiation.